1. Field of the Invention
The present invention relates to an enamel coating method and an enamel coating apparatus using the same, and more particularly, to an enamel coating method and apparatus for a metal pin tube and a manhole cover in which a heat transfer pin is formed on the surface thereof.
2. Description of the Related Art
There are many cases in which a metal tube, a manhole cover, and the like are used in a highly humid environment, and therefore various types of coatings are applied to surfaces of the metal tube, the manhole cover, and the like in order to improve durability thereof. Among these, enamel coating has strong heat resistance and acid resistance characteristics, but is not widely used due to its difficult coating process such as requiring a firing process at a high temperature (750 to 1,000° C.).
In particular, since the firing process at the high temperature is required, it is difficult to secure economic efficiency when preparing a large scale firing chamber for the purpose of coating a long and bulky coating object with enamel. Thus, it is highly difficult to apply the enamel coating to the long and bulky coating object.
However, when coating a metal tube used in a harsh environment (when performing heat exchange using a gas containing acid components in a generator, a tube surface and a heat transfer pin are exposed to an acid dew point while the gas is cooled, and therefore a low temperature corrosion phenomenon occurs in a pin tube) such as a pin tube used as a heat exchanger for generator, with a general coating composition, the corroded metal tube should be replaced periodically, and therefore there are problems that cause inconvenience and high costs. In order to solve these problems, a metal tube made of a stainless steel or titanium-based material is used, but this requires high costs and cannot be the fundamental solution for acid corrosion even though the replacement cycle of the pin tube is increased. In addition, Teflon coating may be applied to the pin tube, but the Teflon material has a limitation in heat resistance. Accordingly, it can be desirable to apply the enamel coating to the metal tube, the manhole cover, and the like which are used in the harsh environment requiring heat resistance and acid resistance.
Coating the surfaces of the metal tube, the manhole cover, and the like with enamel is difficult for the following reasons.
First, as described above, a high-temperature firing chamber is required. In particular, when a length of the metal tube is large, a large-sized high temperature firing chamber is required.
Second, the metal tube is likely to be bent or distorted during coating while being subjected to the high temperature firing process. In this instance, as the length of the metal tube is increased, a frequency of the occurrence of this problem increases. Accordingly, a method for preventing such deformation of the metal tube when coating the metal tube is needed.
Third, oil-based components bonded to the surface of the metal tube in a preprocessing process hinder an aqueous enamel glaze from being bonded to the surface of the metal tube, and therefore it is necessary to remove the oil-based components before applying the enamel coating. However, through the conventional preprocessing processes, it is difficult to efficiently remove the oil-based components.
That is, a blasting process of the preprocessing process cannot efficiently remove the oil-based components, and when a wet process is introduced, the preprocessing process becomes complicated and causes deterioration in coating efficiency. Accordingly, there is a demand for a method for easily removing the oil-based components bonded to the surface of the metal tube.
Fourth, it is difficult to remove air bubbles formed within the coating film after coating with the enamel glaze. Removing the air bubbles formed in the coating film affects the coating quality, and is very important in the coating process. In particular, in a pin tube for a heat exchanger (see FIG. 10) in which a large number of heat transfer pins are formed on the surface of the metal tube, it is difficult to remove air bubbles formed on the surface of the tube and the heat transfer pin, but by efficiently removing such air bubbles, it is possible to manufacture the pin tube having excellent durability. Conventionally, a method of removing the air bubbles by tapping the metal tube has been used, but when coating a long metal tube, such a method is difficult to use and the effect is not significant.
Therefore, in order to take advantage of excellent properties of the enamel coating by applying the enamel coating to the metal tube, the above-described problems should be first solved.